This is more like a snack stand and is located inside the Butterfly building. You can get some street-vendor quality food and other munchies. It's enough to hold you over until you can stop for a "real" meal and it's location is convienent.
A little more upscale than most, but the atmosphere is nice.
From October 24th 2004 to January 6th 2005: Every Sun- and public holiday Brunch from 11 a.m. to 2 p.m.
From November 18th to December 18th 2004: Swedish Christmas Dinner. Thursday to Saturday, from 6.30 p.m.
In the Mainau Autumn and Winter (October 25th to mid-March 2005):
Free admission if lunch or dinner reservations on the island are made; just park at any of the parking lots on the island.
We only stumbled upon one restaurant and it was outdoor seating only. The fare was mediterranean with a touch of Bavarian and it was wonderful. A little on the pricey side, but we were so hot and hungry after walking around all day we weren't really that picky. I have since learned that there were other, cheaper, alternatives.
Use this place for your sweet tooth. Lots of good cakes, tortes, and other confectionaries. There are tables both on the patio and in the castle.